By pigments (Latin pigmentum; paint, makeup) are meant colorants which in the application medium (binders, diluents) are insoluble and in the colored end application are therefore present in the form of finely divided solids (pigment particles). The impression of color they give comes about either through absorption or reflection (remission) of particular frequency components of visible light.
Pigments in accordance with this definition constitute a suspension of pigment particles in a matrix. As they are present in the form of solids, the resulting color effect is affected not only by the basic substance itself but also by its shape, size, surface, and disposition. With conventional pigments, the color effect derives from absorption or reflection of particular wavelengths as a result, for example, of selective excitation of electron transitions in atoms and/or molecules of the pigment material, or by selective excitation of electron vibrations within characteristic functional groups of the pigment material. Regarding the production, processing, and handling of pigments in paints or coatings, for example, or in other coloring formulations, there is extensive prior art known.
Besides the conventional pigments, a large part has been played more recently by luster pigments or effect pigments. With this kind of pigments, the color effect is influenced, or even wholly brought about, by interference or diffraction (scattering). This effect occurs when light is diffracted at regular structures which have dimensions similar to those of the wavelengths of the light. As a result of transit time differences which occur in this case, there may be an attenuation (destructive interference) or intensification (constructive interference) as a function of the wavelength of the light. The structures may be, for example, thin layers of high refractive index, or else regular lines or dots, examples being diffraction gratings. Because of the generation of color by reflection, therefore, it is no longer the inherent color of the pigment that is critical, but rather its structure and the angle of incidence of the light, and also its orientation relative to the viewer. Consequently it is possible to achieve color effects which are not possible with conventional pigments. The more uniform the orientation of the pigments, the more intense the effects are.
Diffractive Pigments
The generation of colors using diffractive elements, such as diffraction gratings, for example, is described in U.S. Pat. No. 3,957,354 or EP 0 632 296, for example. These texts disclose linear patterns which on irradiation with sunlight or another polychromatic light source lead to particular defined color impressions.
Another approach is disclosed by the text DE 199 12 160. In order to produce a colored image or a hologram present in the form of a digitally stored image, a material having a permanently embossable surface is embossed with dots which in each case exhibit a pattern of lines running in parallel, with a distance between them in the range from 100 nm to 2000 nm, depending on the color to be generated. The dots are embossed by means of a needle printer or dot-matrix machine which has a set of needle points for the required base colors.
The use of diffractive structures on pigments or on leafletlike pigments (flakes) is disclosed in DE10252645 A1 or WO2003/011980.
The problem with these structures is that of endowing them with additional optical properties by the application of thin layers, since for this purpose it is necessary to maintain their structure.
Interference Pigments
Interference pigments usually consist of platelet-shaped substrates coated thinly with layers that refract light. Depending on the thickness of the coating, different colors can be produced. Pigments of this kind are also referred to as pearlescent pigments. A substrate commonly used is mica, in various particle sizes. A coating employed is usually TiO2, preferably in rutile modification. Thus, for example, the laid-open specification WO9920695 (Merck) discloses interference pigments based on multiply coated, platelet-shaped substrates, having at least one layer sequence of: (A) a coating having a high refractive index (B) a colorless coating having a low refractive index.
U.S. Pat. No. 3,438,796 and U.S. Pat. No. 5,135,812 describe, for example, metallic luster pigments which have a central opaque aluminum film coated on both sides in alternation with dielectric films of low refractive index, such as silicon dioxide or magnesium fluoride, for example, and with partially transparent metal films, such as chromium or aluminum for example. Because of the production method, the central metal film in these pigments is coated only on the top and bottom faces of the platelets, while the side faces constitute fracture edges and are open toward the medium.
In view of the great importance of interference pigments as luster pigments or effect pigments, there exists an extensive prior art on the single and multiple coating with different materials. In this prior art, there is description not only of coatings for influencing the refraction of light, but also, for example, of protective layers, (e.g., DE 10 2006 009 129 A1, EP 1 727 864 B1) or layers for influencing the orientation of the pigments (EP 1 084 198 B1).
Typical substrates for such pigments include thin platelets of metal oxides, silicates (e.g., mica), but also glass or metal platelets, or even platelets made of polymers.
Besides individual pigments it is also possible for leafletlike platelets (flakes) or whole surfaces to be coated with one or more interfering layers. However, the production of such thin defined layers, more particularly of interference layers, is difficult, since exacting requirements are imposed on the homogeneity of the layer.
At the same time, production by gas-phase deposition is complicated and expensive, and is not very variable in terms of the substances that can be used. The use of sol-gel systems for coating is indeed more versatile, but the production of thin films by the techniques described is difficult.
DE 198 23 732 A1 discloses the use of polymerizable particulate solids for producing optical multilayer systems, but does not describe influencing the surface tension for the purpose of producing thin layers or films, or the production of pigments.
Furthermore, the provision of suitable substrates for interference pigments or flakes or relatively large surfaces with a suitable layer thickness is also a problem. By means of relatively thin substrates it is possible to reduce not only the thickness of the coated pigments but also the thickness of the requisite coat of a paint comprising these pigments. In this context it is also important that a production method for substrates is particularly versatile, in order to provide a broad spectrum of substrates, for different colors or refractive indices, for example.
Problem
The problem addressed by the present invention is to specify a method which allows the production of films, more particularly for the coating of surfaces, more particularly of pigments or flakes. The method ought ideally to be suitable at the same time for producing pigments, flakes or sheets. In this context, the method ought not only to be versatile in terms of the materials or substrates which can be used, but also easy to implement and inexpensive.